Anisotropic Touch Screen Element

ABSTRACT

A touch sensitive position sensor for detecting the position of an object in two dimensions is described. The position sensor has first and second resistive bus-bars spaced apart with an anisotropic conductive area between them. Electric currents induced in the anisotropic conductive area by touch or proximity flow preferentially towards the bus-bars to be sensed by detection circuitry. Because induced currents, for example those induced by drive circuitry, flow preferentially along one direction, pin-cushion distortions in position estimates are largely constrained to this single direction. Such one-dimensional distortions can be corrected for very simply by applying scalar correction factors, thereby avoiding the need for complicated vector correction.

BACKGROUND OF THE INVENTION

The invention pertains to 2-dimensional touch sensing surfaces operableby a human finger, or a stylus. Example devices include touch screensand touch pads, particularly those over LCDs, CRTs and other types ofdisplays, or pen-input tablets, or encoders used in machinery forfeedback control purposes. Descriptions of pen or touch input to amachine date back to at least 1908, as embodied in patent DE 203,719.

Touch screens and pointing devices have become increasingly popular andcommon not only in conjunction with personal computers but also in allmanner of other appliances such as personal digital assistants (PDAs),point of sale (POS) terminals, electronic information and ticketingkiosks, kitchen appliances and the like. These devices are evolvingcontinuously into lower priced products and as a result, there is a needfor ever lower production cost while maintaining high levels of qualityand robustness. Capacitive touch screens in particular are prized fortheir robustness against damage, but suffer from high costs and the needfor exotic construction materials.

The term ‘two-dimensional capacitive transducer’ or ‘2DCT’ will be usedthroughout to refer to touch screens, touch sensing pads, proximitysensing areas, display overlay touch screens over LCD, plasma, or CRTscreens or the like, position sensing for mechanical devices or feedbacksystems, or other types of control surfaces without limitation, having asurface or volume capable of reporting at least a 2-dimensionalcoordinate, Cartesian or otherwise, related to the location of an objector human body part, by means of a capacitance sensing mechanism.

The term ‘two-dimensional resistive transducer’ or ‘2DRT’ will be usedthroughout to refer to touch screens or pen input devices based onpurely galvanic principles, and known in the industry generically andprimarily as ‘resistive touch screens’.

The term ‘2DxT’ refers to elements of either the 2DCT or 2DRT type.

The term ‘touch’ throughout means touch or proximity by a human bodypart or mechanical component of sufficient capacitive signal strength togenerate a desired output. In the sense of ‘proximity’, touch can alsomean to ‘point’ at a 2DCT without making physical contact, where the2DCT responds to the capacitance from the proximity of the objectsufficient to react properly.

The term ‘element’ throughout refers to the active sensing element of a2DCT or 2DRT. The term ‘electrode’ refers to a connection point at theperiphery of the element.

The term ‘stripe’ refers to an electrical line conductor that is acomponent part of an element and which has two ends. A stripe can be awire. A stripe can have substantial galvanic resistance by intent,whereas a wire has minimal resistance. If the element of which it is apart is physically curved, the stripe would also be physically curved.

The term ‘pin cushion’ refers to any distortion of the signal from a2DCT whether parabolic, barrel, or other form of 2D dimensionalaberration.

Many types of 2DCT are known to suffer from geometric distortioncharacterized as ‘pin cushion’ or ‘hyperbolic’ or ‘parabolic’, wherebythe reported coordinate of touch is in error due to electrical effectson the sensing surface. These effects are described in more depth invarious other patents for example in Pepper U.S. Pat. No. 4,198,539,incorporated by reference. An excellent summary of the known causes,solutions, and problems of the solutions to geometric distortion can befound in a reading of Babb et al, in U.S. Pat. No. 5,940,065, and U.S.Pat. No. 6,506,983, incorporated by reference. U.S. Pat. No. 5,940,065describes succinctly the two major classes of correction: 1)Electromechanical methods involving design of or modification to thesensing surface or the connecting electrodes; 2) Modeling methods usingmathematical algorithms to correct the distortions.

Electromechanical Methods

Edge Manipulation of Planar Element: Küpfmüller et al in U.S. Pat. No.2,338,949 (filed 1940) solve the problem of edge distortion in a 2DRTelectrograph using very long rectangular tails in X and Y surrounding asmall usable area Küpfmüller takes the further approach of slotting thefour tails into stripes; these stripes do not intrude on the user inputarea but do act to raise the resistance to current flow in ananisotropic manner along sides parallel to current flow. This ideareappears in slightly different form in Yaniv et al, U.S. Pat. No.4,827,084, nearly 50 years later. Küpfmüller remains the most similarprior art to the instant invention.

Becker in U.S. Pat. No. 2,925,467 appears the first to describe a 2DRTelectrograph whereby nonlinear edge effects are eliminated via the useof a very low resistance edge material relative to the sheet resistanceof the element proper. This method can also be used to construct a 2DCT.

Pepper, in patents U.S. Pat. No. 4,198,539, U.S. Pat. No. 4,293,734 ,and U.S. Pat. No. 4,371,746 describes methods of linearizing a 2DCT bymanipulating the edge resistance structure of the element.

Talmage, in U.S. Pat. No. 4,822,957 describes a similar edge pattern asPepper in conjunction with a 2DRT element and a pickoff sheet. Numerousother such patents have been issued using various methods, and the arearemains a fertile one for new patents to this day. These methods havebeen found to be very difficult to develop and replicate, and they areprone to differential thermal heating induced errors and productionproblems. Very small amounts of localized error or drift can causesubstantial changes in coordinate response. The low resistance of thepatterned edge strips causes problems with the driver circuitry, forcingthe driver circuitry to consume more power and be much more expensivethan otherwise. There are a significant number of patents that referencethe Pepper patents and which purport to do similar things. Theimprovements delivered by Pepper etc over Becker are arguably marginal,as at least Becker is easier and more repeatable to fabricate.

Edge Resistance with Wire Element: Kable in U.S. Pat. No. 4,678,869discloses a 2D array for pen input, using resistive divider chains on 2axes with highly conductive electrodes connected to the chains, theelectrodes having some unintended resistance for the purposes ofdetection, and the detection signal being interpolated from the signalsgenerated between two adjacent electrodes. The unintended resistancecauses a slight amount of pin cushion in the response. This patent alsodescribes an algorithmic means to compensate for the slight pin-cushiondistortion developed by this technique. The Kable method is not operablewith other than a connected stylus, i.e. it is not described as beingresponsive to a human finger. The Kable patent requires crossoversbetween conductors and thus needs at least three construction layers(conductor, insulator, conductor).

Multiple Active-Edge Electrodes: Turner in U.S. Pat. No. 3,699,439discloses a uniform resistive screen with an active probe havingmultiple electrode connections on all four sides to linearize theresult.

Yoshikawa et al, in U.S. Pat. No. 4,680,430, and Wolfe, in U.S. Pat. No.5,438,168, teach 2DCT's using multiple electrode points on each side (asopposed to the corners) to facilitate a reduction in pin cushion byreducing the interaction of the current flow from the electrodes on oneaxis with the electrodes of the other. While the element is a simplesheet resistor, this approach involves large numbers of activeelectronic connections (such as linear arrays of diodes or MOSFETs) ateach connection point in very close proximity to the element.

Nakamura in U.S. Pat. No. 4,649,232 teaches similarly as Yoshikawa andWolfe but with a resistive pickup stylus.

Sequentially Scanned Stripe Element: Greanias et al in U.S. Pat. No.4,686,332 [16] and U.S. Pat. No. 5,149,919, Boie et al in U.S. Pat. No.5,463,388, and Landmeier in U.S. Pat. No. 5,381,160 teach methods ofelement sensing using alternating independently driven and sensed stripeconductors in both the X and Y axis, from which is interpreted aposition of a finger touch or, by a pickup device, a stylus pen. Theconstruction involves multiple layers of material and specialprocessing. Greanias teaches the use of interpolation between thestripes to achieve higher resolution in both axis. Both require three ormore layers to allow crossovers of conductors within the element. Bothrely on measurements of capacitance on each stripe, not the amount ofcross coupling from one stripe to another. Boie also teaches a specialguard-plane.

Binstead, in U.S. Pat. No. 5,844,506 and U.S. Pat. No. 6,137,427 teachesa touch screen using discrete fine wires in a manner similar to thosetaught by Kable, Allen, Gerpheide and Greanias. Binstead uses very finerow and column wires to achieve transparency. This patent also teachesthe Greanias method of interpolation between electrode wires to achievehigher resolution. The scanning relies on measurements of capacitance oneach stripe to ground, not the amount of cross coupling from one toanother.

Evans in U.S. Pat. No. 4,733,222 also describes a system wherein stripesare sequentially driven in both X and Y axis, using also an externalarray of capacitors to derive sensing signals via a capacitor dividereffect. Interpolation is used to evaluate finer resolutions thanpossible with the stripes alone.

Volpe in U.S. Pat. No. 3,921,166 describes a discrete key mechanicalkeyboard that uses a capacitive scanning method. There are sequentiallydriven input rows and sequentially sensed columns. The press of a keyincreases the coupling from a row to a column, and in this way n-keyrollover can be achieved; there is no need for interpolation. Althoughnot a 2DCT, Volpe presages scanned stripe element 2DCT technology. Myown U.S. Pat. No. 6,452,514 also falls into this classification ofsensor.

Itaya in U.S. Pat. No. 5,181,030 discloses a 2DRT having resistivestripes which couple under pressure to a resistive plane which reads outthe location of contact. The stripes, or the plane, have a 1D voltagegradient imposed on them so that the location of contact on particularthe stripe can be readily identified. Each stripe requires its own, atleast one electrode connection.

Cyclical Scanned Stripe Element: Gerpheide et al, in U.S. Pat. No.5,305,017 teaches a touch-pad capacitance-based computer pointing deviceusing multiple orthogonal arrays of overlapping metallic stripesseparated by insulators. The scan lines are arranged in a cyclicallyrepeating pattern to minimize drive circuitry requirements. A cyclicalnature of the wiring of the invention prevents use of this type of 2DCTfor absolute position location. The invention is suited to touch padsused to replace mice, where actual location determination is notrequired, and only relative motion sensing is important. Gerpheideteaches a method of signal balance between two phase-opposed signals atthe location of touch.

Parallel Read Stripe Element: Allen et al in U.S. Pat. No. 5,914,465teach an element having rows and column scan stripes which are read inparallel by analog circuitry. The patent claims lower noise and fasterresponse times than sequentially scanned elements. The method isparticularly suited to touch pads for mouse replacement but does notscale well to higher sizes. Multiple construction layers are required aswith all stripe element 2DCT's. The Allen method requires large scaleintegration and high numbers of connection pins. It interpolates toachieve higher resolution than achievable by the number of raw stripes.

In my co-pending U.S. application Ser. No. 10/697133, “Charge TransferCapacitive Position Sensor” there is described in conjunction with FIG.12 a method of using individual resistive 1-D stripes to create a touchscreen. These stripes can be read either in parallel or sequentially,since the connections to these stripes are independent of one another.Furthermore, in connection with FIG. 6 there is described aninterpolated coupling between adjacent lumped electrode elements and anobject such as a finger. U.S. application Ser. No. 10/697133 isincorporated herein by reference.

Numerical Methods

Nakamura in U.S. Pat. No. 4,650,926 describes a system for numericalcorrection of an electrographic system such as a tablet, using a lookuptable system to correct raw 2D coordinate data.

Drum, in U.S. Pat. No. 5,101,081 describes a system for numericalcorrection of an electrographic system such as a tablet via remotemeans.

McDermott in U.S. Pat. No. 5,157,227 teaches a numerical method ofcorrecting a 2DxT employing stored constants which are used duringoperation to control one or more polynomials to correct the location ofreported touch by zone or quadrant.

Babb et al, in U.S. Pat. Nos. 5,940,065 and 6,506,983 teach a numericalmethod to linearize a 2DxT uniform sheet element using coefficientsdetermined during a learn process, without segmentation by zone orquadrant, and on an individual unit basis so as to correct for evenminor process variations. The methods disclosed by Babb are complex andinvolve ‘80 coefficients’ and fourth order polynomials, the coefficientsof which must be determined through a rigorous and time-consumingcalibration procedure. In tests supervised by the instant inventor, ithas been found that 6^(th) order polynomials are required to produceaccuracy levels that are acceptable in normal use, and that the resultis still highly prone to the slightest subsequent variationspost-calibration due to thermal drift and the like. In particular it hasbeen found that the corner connections are extreme contributors tolong-term coordinate fluctuations, as they act as singularities with ahigh gain factor with respect to connection size and quality.Furthermore, the method of numerical correction requires high-resolutiondigital conversions in order to produce even modest resolution outputs.For example it has been found that a 14-bit ADC is required to provide aquality 9-bit coordinate result. The extra expense and power required ofthe amplifier system and ADC can be prohibitive in many applications.

Technology Summary

In all these methods there exists one or a combination of the followingdeficiencies:

-   Use of exotic construction materials or methods requiring special    expertise or equipment to fabricate;-   Excessive cost compared with simple, galvanic 4-wire resistive touch    screens;-   Require three or more layers to allow orthogonal conductor    crossovers;-   Costly wiring due to the need for many electrode connections;-   Linearity problems requiring complex algorithms to correct;-   Need for special linearizing edge patterns which are difficult to    control;-   Not well suited to small or large touch areas;-   Inability to conform to complex surface shapes such as compound    curves; and/or-   Inability to operate through surfaces more than a few hundred    microns thick.

SUMMARY OF THE INVENTION

According to the invention there is provided a touch sensitive positionsensor comprising: a substrate defining a touch sensitive platform;first and second resistive bus-bars arranged spaced apart on thesubstrate; and an anisotropic conductive area arranged between thebus-bars such that currents induced in the anisotropic conductive areaflow preferentially towards the bus-bars.

In typical embodiments of the invention, the bus-bars and theanisotropic conductive area have resistances of between 1 kΩ and 50 kΩ.The bus-bars preferably have substantially the same resistance, forexample to within ±10%, 20%, 50% or 100%. It is advantageous if theresistance of the bus-bars is less than the resistance between themprovided by the anisotropic conductive area.

The anisotropic conductive area can be fabricated using a film ofmolecular substance having anisotropic conduction supported on asubstrate, or a plurality of resistive stripes connecting in parallelbetween the first and second resistive bus-bars, or in other ways.

When resistive stripes are used to form the anisotropic conductive areathese can be made of sections of resistive wire, or from resistivematerial deposited on a substrate, for example. Moreover, the width ofthe resistive stripes is preferably greater than the gaps between them.

In some embodiments of the invention, a conductive overlay is providedthat is separated from the anisotropic conductive area such that theconductive overlay and the anisotropic conductive area may be broughtinto contact by externally applied pressure.

In some embodiments, the first resistive bus-bar extends between a firstand a second electrode and the second resistive bus-bar extends betweena third and a fourth electrode, the position sensor further comprisingfirst, second, third and fourth drive channels associated withrespective ones of the first, second, third and fourth electrodes, eachdrive channel being operable to generate an output signal dependent onthe resistance between its electrode and the position of the object. Forprocessing the outputs, a processor may be provided that is operable togenerate an estimate for the position of the object by comparing theoutput signals from the drive channels. The processor can be configuredto estimate the position of the object in a first direction runningbetween the bus-bars from a ratiometric analysis of the sum of thesignals associated with the first and second electrodes and the sum ofthe signals associated with the third and fourth. It can also beconfigured to estimate the position of the object in a second directionrunning along the bus-bars from a ratiometric analysis of the sum of thesignals associated with the first and third electrodes and the sum ofthe signals associated with the second and fourth electrodes. Moreover,the processor is preferably further operable to apply a correction tothe estimated position according to a pre-determined distortionassociated with the sensing element. Typically, the pre-determineddistortion is a one-dimensional pin-cushion distortion.

It will be understood that a touch sensitive position sensor accordingto the invention can be incorporated into a control panel and in turnthe control panels can be integrated as part of a variety of differentapparatuses.

According to the invention there is also provided a touch sensitiveposition sensor for detecting the position of an object in twodimensions, wherein the position sensor has first and second resistivebus-bars separated by an anisotropic conductive area, the anisotropicconductive area being arranged such that induced electric currents flowpreferentially towards the bus-bars. Because induced currents, forexample those induced by drive circuitry associated with the sensingelement, flow preferentially along one direction, pin-cushion typedistortions in position estimates are largely constrained to thisdirection. Such one-dimensional distortions can be corrected for byapplying scalar correction factors.

The invention provides a new pattern of conductive material for sensingcapacitance behind a plastic or glass panel or other dielectric, whichis to be used as a 2DxT, whether in the format of a touch screen or‘touch pad’.

The invention blends some of the features of unpatterned 4-electrodeelements together with striped elements and mathematical compensation toarrive at a new classification of anisotropic 2DxT element, or simply, a‘striped element’. This invention addresses the deficiencies of previous2DxT approaches and is very low in cost, using as it does conventionalprocesses and materials.

Unless otherwise noted hereinafter, the terms ‘connection(s)’ or‘connected’ refer to either galvanic contact or capacitive coupling.‘Element’ refers to the physical electrical sensing element made ofconductive substances. ‘Electrode’ refers to one of the galvanicconnection points made to the element to connect it to suitabledriver/sensor electronics. The terms ‘object’ and ‘finger’ are usedsynonymously in reference to either an inanimate object such as a wiperor pointer or stylus, or alternatively a human finger or otherappendage, any of whose presence adjacent the element will create alocalized capacitive coupling from a region of the element back to acircuit reference via any circuitous path, whether galvanically ornon-galvanically. The term ‘touch’ includes either physical contactbetween an object and the element, or, proximity in free space betweenobject and element, or physical contact between object and a dielectric(such as glass) existing between object and element, or, proximity infree space including an intervening layer of dielectric existing betweenobject and element. The mention of specific circuit parameters, ororientation is not to be taken as limiting to the invention, as a widerange of parameters is possible using no or slight changes to thecircuitry or algorithms; specific parameters and orientation arementioned only for explanatory purposes.

Note my prior patents covering charge-transfer capacitive sensing,particularly U.S. Pat. No. 5,730,165, U.S. Pat. No. 6,288,707, U.S. Pat.No. 6,466,036, U.S. Pat. No. 6,535,200, U.S. Pat. No. 6,452,514 and myco-pending U.S. application Ser. No. 10/697133. In particular it shouldbe noted that the electronic sensing circuitry and methods described ineach of these patents can be used in conjunction with the inventiondescribed herein, but, these methods are not the only ones. A variety ofcapacitive sensing circuits can be used with the invention. Variouselectrical circuits and sensing methods described in these patents canbe employed to drive the electrodes of the invention and to interpretthe results.

Note also my U.S. application Ser. No. 10/341948 (US 20030132922) whichdeals with handshadow effects on capacitive touchscreens, and which hasa possible application to the invention in a post-processing role for2DCT's.

My co-pending patent application U.S. Ser. No. 10/697133, “ChargeTransfer Capacitive Position Sensor” in particular as described inconjunction with FIG. 12 therein, forms a germinal basis for theinvention, and whose circuit description and switching methods areparticularly well adapted to drive the electrodes of the invention in a2DCT mode. The invention is a new pattern of conductive material, suchas an ink or vacuum deposited material, arranged electrically as asingle layer element, with pin-cushion distortion on only one axis. Theremaining pin-cushion distortion is easily corrected algorithmically orin hardware, vastly simpler than Babb & Wilson, as will be describedbelow. The element pattern is easily fabricated using known methods andis conformable to complex surfaces such as compound curved cover lensesand the like. The pattern exhibits strong anisotropic conductancecharacteristics in a core sensing region bounded by peripheralunidirectional resistive conductors.

It is one object of the invention to provide for a 2DxT sensing elementusing common, inexpensive materials and production processes, withanisotropic galvanic conduction characteristics.

It is a further object of the invention to provide a 2DxT sensingmechanism having an edge distortion that is readily correctable usingsimple, computationally inexpensive methods.

It is an object of the invention to permit position interpolation so asto achieve the highest possible resolution with the simplest possiblepattern.

It is another object of the invention to provide a 2DxT element allowinga high positional resolution and low granularity result with relativelycoarse raw signal analogue-to-digital converter (ADC) resolution.

Another object is to provide a 2DxT element that is less susceptible tothermal drift, and is highly repeatable in the manufacturing process.

Another object of the invention is to provide a 2DxT element that eitherrequires a highly simplified ‘learn’ calibration process compared withthe prior art, or, calibration via design, or, none at all.

Another object is to provide for a 2DCT element having only one requiredlayer of conductive material.

A further object is to allow this layer to reside on the rear ofrelatively thick dielectric cover lenses such as glass or plastic sheet,up to 10 mm in thickness or more, or through air by pointing.

A further object of the invention to provide a 2DxT element havingrelatively simple wiring requirements;

Further objects of the invention are to provide for a sensor having highreliability, a sealed surface, low power consumption, and the ability tobe controlled and sensed directly using off-the-shelf microcontrollersand non-exotic drive electronics.

Further particular and preferred aspects of the invention are set out inthe following non-limiting independent and dependent clauses:

-   -   1. Apparatus of a type wherein a surface is selectively accessed        with respect to positional data, comprising a conductive element        having a core with a direction of preferential galvanic        conduction.    -   2. The apparatus of clause 1 wherein the element is bounded by a        conductive border    -   3. Clauses 1 or 2 wherein the element resides on a single layer    -   4. Clauses 1 or 3 including a plurality of electrodes    -   5. Any preceding clause including circuitry connected to said        element for the purpose of evaluating the location of touch in        two dimensions, where the connections are made to the        electrodes.    -   6. Any preceding clause including processing means to correct        for pin-cushion distortion    -   7. Clause 6 wherein the correction is a scalar coefficient.    -   8. Clause 6 wherein the correction is based on a set of scalar        coefficients    -   9. Clause 6 wherein the correction is based on a formula of the        type        ${Pcy}_{({x,y})} = \frac{P_{Y}}{{k_{1}X^{2}} + {k_{2}X} + k_{3}}$    -   10. A method of fabricating an element used for the        determination of positional location of touch, whereby the        element is made to have anisotropic conductivity with a        conductive perimeter.    -   11. Clause 10 where the element is made from an anisotropic        material.    -   12. Clause 10 including a method to correct for positional        distortion    -   13. Clause 12 wherein the method for correction of distortion is        only applied to one axis.    -   14. Clause 12 wherein the method for correction of distortion is        based on scalar multiplication.    -   15. Any preceding clause whereby the electronic circuitry        employs rail-referenced charge-sensing according to any method        disclosed in my U.S. Pat. No. 6,466,036 [34].    -   16. Clause 15 whereby the circuitry comprises a microcontroller.    -   17. Any preceding clause whereby the element is made from an        optically transmissive resistive conductor.    -   18. Any preceding clause whereby the element comprises a        plurality of zones of anisotropic conductance sharing common        bus-bars.    -   19. A touchscreen having an optically transmissive element of        anisotropic conductance, affixed to the distal side of an        optically transmissive substrate, the proximal side being used        for touch, having a plurality of electrodes.    -   20. Clause 19 whereby the electrodes are connected to a sensing        circuit using conductive rubber.    -   21. Clause 19 or 20 whereby the touchscreen is mounted over an        electronic display.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention and to show how the same maybe carried into effect reference is now made by way of example to theaccompanying drawings in which:

FIG. 1 a schematically shows typical pin-cushion distortion effectsfound in unpatterned, single element two-dimension transducers made froma resistive film having four corner electrodes and a ‘pickoff’ flexiblecover sheet according t the prior art;

FIG. 1 b schematically shows the normalization vectors required tolinearize the element of FIG. 1 a;

FIG. 2 shows a known capacitive or resistive touch screen edge patterndesigned to correct pin cushion effects in screens suffering fromdistortions of the kind shown in FIGS. 1 a and 1 b;

FIG. 3 schematically shows a two-dimensional pattern representative ofthe conductive material used to form a sensing element according to anembodiment of the invention;

FIG. 4 schematically shows an electrical circuit representation of thesensing element of FIG. 3;

FIG. 5 schematically shows the sensing element of FIG. 3 with thelocation of a touch identified;

FIG. 6 schematically shows a vertical section the sensing element ofFIG. 5 taken at the location of the touch;

FIG. 7 a schematically shows a row-by-row linearity plot in one quadrantof the sensing element of FIGS. 3 and 5;

FIG. 7 b schematically shows the distortion associated with the sensingelement of FIGS. 3 and 5;

FIG. 8 schematically shows normalization vectors required to linearizethe distortion shown in FIGS. 7 a and 7 b;

FIG. 9 schematically shows a two-dimensional pattern representative ofthe conductive material used to form a sensing element according toanother embodiment of the invention;

FIG. 10 schematically shows a two-dimensional pattern representative ofthe conductive material used to form a sensing element according to yetanother embodiment of the invention;

FIG. 11 schematically shows a vertical section the sensing element ofFIG. 10 forming part of a resistive touch screen;

FIG. 12 schematically shows a two-dimensional pattern representative ofthe conductive material used to form a sensing element according to yetanother embodiment of the invention;

FIG. 13 schematically shows the sensing element of FIG. 5 having coupledto drive channels based on charge-transfer methods;

FIG. 14 schematically shows a processor arranged to receive signalsoutput from the drive channels of FIG. 13 and to calculate an estimatedposition of a touch therefrom;

FIG. 15 schematically shows a microcontroller connected to four samplingcapacitors, the microcontroller and capacitors being configured toprovide the sensing channels and processor of FIGS. 13 and 14;

FIG. 16 schematically shows in vertical section a capacitive basedposition sensor according to an embodiment of the invention arrangedover a liquid crystal display so as to create a touch-sensitive screen;

FIGS. 17 to 19 schematically shows two-dimensional patternsrepresentative of the conductive material used to form sensing elementsaccording to further embodiments of the invention;

FIG. 20 schematically shows a sensing element incorporating theresistive pattern of FIG. 19 when in use; and

FIG. 21 schematically shows a two-dimensional patterns representative ofthe conductive material used to form a sensing element according toanother embodiment of the invention.

DETAILED DESCRIPTION

FIGS. 1 a and 1 b show the prior art for 2DxT technology prior to theuse of correction hardware or algorithms. The pin cushion effect of FIG.1 a is well understood. It arises from the current sharing ofcapacitance-induced flows from the point of touch to the four connectionpoints; the effect is seen in both 2DCT's and in 2DRT 5-wire touchscreens which rely on a galvanic version of the same voltage gradientsas a 2DCT, but with a flexible ‘pickoff’ cover sheet that deflects andconnects to the 2DRT under pressure. The pin cushion effect in theseelements increases as the location of touch becomes more distant fromall connection points, along an edge; it is at its worst at the centersof the screen edges. As shown in FIG. 1 b, the current flows establishvectors that introduce a graduated distortion with position, resultingin a parabolic curvature of reported location. The vectors are generallynon-orthogonal. Instead the angle and magnitude of correction varywildly depending on the location of touch on the element.

Various methods have been devised to counter this effect, notably theuse of very low resistance bus-bars around the conductive screen,special edge patterns, multiple connection points to the edges of thescreen, and so on, as described above. Discrete conductors, as seen inBinstead, Gerpheide, Kable and Greanias largely solve the problem of pincushion by using exotic construction methods using multiple layers,expensive circuits, and a high electrode connection count. These typesof screens do not scale well with size and are expensive to fabricate.An example of such a method is the edge pattern devised by Pepper whichis shown in FIG. 2. This pattern is known to be very difficult toduplicate, suffers from thermal drift, and is relatively expensive toengineer and fabricate.

There is a substantial demand for a new capacitive touch screen methodthat is less expensive and simpler to manufacture than the abovemethodologies yet is highly robust and suitable for use in hostileenvironments. In particular there is a need for such devices in theapplications of domestic appliances, mobile phones and other hand-helddevices, POS terminals, and so on.

Embodiments of the invention provide a compromise between the ‘no pincushion’ but expensive circuitry and fabrication cost of prior-artstriped elements, and unpatterned resistive sheet elements. This newhybrid solution produces a pin cushion effect only on one axis, leavingthe other axis largely undistorted. Furthermore, as will be seen, theresidual pin-cushion distortion has a largely orthogonal and predictablevector which can be compensated using relatively trivial numericalmethods, is highly repeatable from unit to unit, and is more immune todifferential thermal drift than the prior art.

In FIG. 3 is shown a pattern representative of the conductive materialused in a sensing element according to an embodiment of the invention.The diagram shows a single conductive element on one layer having fourelectrodes 301, 302, 303, and 304. Two relatively low resistancebus-bars 305 and 306 traverse from 301 to 302, and 304 to 303respectively. A plurality of stripe conductors 310 traverse from bus-bar305 to 306, numbering at least two but typically 3 or more. Two of thesestripes traverse from the ends of each bus-bar to the other, thusforming a fully bounded surface. The end stripes can also be consideredto be bus-bars, but as they can optionally have a higher path resistancethan the horizontal bus-bars shown, they remain unique and thus will becalled stripes throughout.

The element of the invention can alternatively be viewed as having acore area characterized by anisotropic conductivity with a surrounding,bounding border made from linear conductive segments. The purpose of thestripes is to force anisotropic galvanic flows within the core area.Once the current flows reach the boundary paths, they are finally led tothe electrode connections.

The number of stripes 310 appropriate for a design depends on the widthof the element in relation to the size of the object being sensed, aswill be discussed below. Wires 312 a-d connected to the electrodesconnect the element to a drive/sensing circuit in the case of a 2DCT. Inthe case of a 2DRT wires 312 a-d are connected to a drive circuit, thesensing function coming from a flexible user-depressed cover sheet asshown in FIG. 11.

FIG. 10 shows another pattern embodying the invention. This pattern issubstantially the same as the pattern of FIG. 3 except that the stripesare separated by thin slits (i.e. the stripes are relatively wider thanthose shown in FIG. 3), so that the element is principally coated withconductive material and only a very small percentage—the slits—isuncoated. This configuration is more suitable for 2DRT use as describedfurther below, but can also be used in 2DCT applications. One advantageof this for 2DCT use is that the stripes have larger surface areas thanin the FIG. 3 example, so that the capacitive coupling from finger toelement is enhanced. One disadvantage of this is that the totalresistance from bus-bar to bus-bar is lower for a particular sheetresistance, which will tend to exacerbate the pin cushion effect asdescribed below.

The relative resistances of the stripes and bus-bars in FIG. 3 as testedare about 40K ohms for the bus-bars, and 160K ohms for the stripes,although in practice these figures are only for guidance and they arenot limiting to the invention. The use of higher resistance valuestripes than bus-bars is helpful to limit pin cushion effects, but sincepin cushion is easily correctable numerically anyway, almost anycombination of values will work to varying degrees of satisfaction. Itis a considerable attraction of the invention that it is usable withelements having a high resistance, as such an element requires lowercost and lower power drive and sensing electronics.

FIG. 4 shows a lumped model of typical embodiments of the invention.Bus-bars 305 and 306 are composed of lines with a resistance from about1K to 50K ohms, and ideally are matched to within 10% of each other.Stripes 310 are composed of resistive lines of about 5 to 10 times morethan the resistance of the bus-bars. There are 9 stripes shown in FIG.4. Corner electrodes 301, 302, 303, and 304 are used to connect theelement to drive/sensing electronics, either capacitive sensing driversfor a 2DCT or galvanic drivers in the case of a 2DRT. Each stripe andbus-bar has some stray background capacitance 401 to circuit ground.Stripes have mutual capacitance 404 between neighbors. Such backgroundcapacitances are benign in nature and have been show to have no effecton the performance of the invention. These capacitances do not have tobe equal or balanced for the invention to work, as the element obeys thephysics of superposition, and such parasitic values are easilycalibrated away by the drive electronics as will be described below.

Shown is a capacitance Ct, 402, at position 403 due to a touch in 2DCTmode. The invention is fully tolerant of the magnitude of Ct, in that itallows the use of circuitry and/or algorithms that respondsratiometrically to the four electrode signals to derive a positionindependent of the magnitude of Ct. In 2DRT mode, the coversheet picksoff a gradient potential, usually using time-multiplexed drive signalsto the four electrodes upon galvanic connection from the coversheet tothe element under the pressure of touch.

In 2DCT mode it is also possible to have a touch between stripes and tointerpolate the location of touch. FIG. 5 shows the element with a touchcapacitance geographically located at 403 due to finger (not shown).FIG. 6, in which is shown a cross section of the invention attached to asubstrate such as glass. The capacitance 603 due to touch of finger 605is split into three smaller parts, Ct1, Ct2, and Ct3 as shown in FIG. 6,whose ratio depends on the relative location of the touch among thestripes 310 a,b,c. In FIG. 6 of my co-pending U.S. application Ser. No.10/697133 is shown an interpolation between two adjacent electrodesconnected by a resistance. The interpolation of touch in the instantinvention operates in exactly the same manner among stripes in X, butalso occurs along each stripe in the Y axis (not shown). The separatingresistance in X is the path starting on each stripe with each Ct 603,back through the bus-bars to the other stripe. The interpolation in theX direction is proportioned according to the resistance of the shortsegment of bus-bar resistance connecting the two Y stripes, as apercentage of the total electrical bus-bar ‘length’. The resistance ofthe stripe itself is not of consequence for resolving X location, sincethe ends of each stripe are driven to equipotentials in most 2DCT drivecircuits described in the literature, and certainly when driven by thecharge-transfer circuits described in my various earlier patentpublications. Thus, if the stripes are spaced apart by 10% of the totalbus-bar length, then the opportunity for interpolation will be 10% ofthe X dimension.

Note that the element of FIG. 5 could be rotated through 90 degrees andthe above discussion would have been in regard to the Y dimension. Thereis no preferred angle of orientation of the element of the inventionwith regard to detection and location of touch. The discussions andformulae noted below are based on an assumption of convenience i.e. thatthe stripes are aligned in a vertical, Y orientation; however, rotationof the element through 90 degrees would provide identical physicaloperation and the equations would still hold albeit translated through90 degrees. Specificity in regard to the orientation in this patent isnot intended except as a matter of explanatory convenience and shouldnot be held to be limiting in anyway.

The measurement circuitry, well described in the literature from avariety of inventors but preferably of any type as disclosed by theinventor in his U.S. Pat. Nos. 6,288,707, 6,466,036 and co-pendingapplication Ser. No. 10/697133, is used by standard connection to thefour corner electrodes 301, 302, 303, and 304. The measurement circuitrycomprises four drive channels coupled to respective ones of theelectrodes shown in FIG. 3 with each channel being operable to generatean output signal dependent on the resistive path length between itselectrode and the position of the touch. While other methods might useother formulae, the preferred method of calculation of the position oftouch is an adaptation of the one disclosed in my co-pending applicationSer. No. 10/697133. In this method the four corner signals arecalibrated at some time to determine a baseline reference level ofsignal for each corner. The calibration step can occur once, for exampleduring design, on the production line, or at each power-up event, orthrough a method that determines when the element is not being touched.Drift compensation can be applied as it is known from several of myprior patents and the datasheets of products from Quantum Research GroupLtd (UK), such as the QT110 device.

To compute the position of touch along X (i.e. the horizontal directionshown in FIG. 3) using the element of FIG. 3 the signals are processedaccording to the following steps assuming that the real time acquiredsignals associated with the four electrodes 301, 302, 303 and 304 arerespectively S301, S302, S303, and S304, and the baseline referencelevels are R301, R302, R303, and R304, respective to each corner:

1) Sum the references and signals in X:RX′=R301+R304 (sum of left references)RX″=R302+R303 (sum of right references)SX′=S301+S304 (sum of left signals)SX″=S302+S303 (sum of right signals)

2) Compute the delta signals in X, i.e. ΔSigX′, ΔSigX″:ΔSigX′=SX′−RX′ΔSigX″=SX″−RX″

3) Compute the ratio Px indicative of position in X:Px=ΔSigX″/(ΔSigX′+ΔSigX″)where the Px is in range of 0 . . . 1, ‘0’ being the left edge, ‘1’ theright edge. The formula for X can be re-expanded to: $\begin{matrix}{P_{x} = \frac{{S\quad 302} + {S\quad 303} - {R\quad 302} - {R\quad 303}}{\begin{matrix}{{S\quad 301} + {S\quad 302} + {S\quad 303} + {S\quad 304} -} \\{{R\quad 301} - {R\quad 302} - {R\quad 303} - {R\quad 304}}\end{matrix}}} & \left( {{Equation}\quad 1} \right)\end{matrix}$

To compute the position of touch along Y using the element of FIG. 3 thesignals are processed according to a formula similar to that indicatedabove:

1) Sum the references and signals in Y:RY′=R303+R304 (sum of bottom references)RY″=R301+R302 (sum of top references)SY′=S303+S304 (sum of bottom signals)SY″=S301+S302 (sum of top signals)

2) Compute the delta signals in Y, i.e. ΔSigY′, ΔSigY″:ΔSigY′=SY′−RY′ΔSigY″=SY″−RY″

3) Compute the ratio Py indicative of position in Y:Py=ΔSigY″/(ΔSigY′+ΔSigY″)where the Py is in range of 0 . . . 1, ‘0’ being the bottom edge, ‘1’the top edge.

The formula for Py can be re-expanded to: $\begin{matrix}{P_{y} = \frac{{S\quad 301} + {S\quad 302} - {R\quad 301} - {R\quad 302}}{\begin{matrix}{{S\quad 301} + {S\quad 302} + {S\quad 303} + {S\quad 304} -} \\{{R\quad 301} - {R\quad 302} - {R\quad 303} - {R\quad 304}}\end{matrix}}} & \left( {{Equation}\quad 2} \right)\end{matrix}$

The complete, reported but uncorrected or ‘raw’ estimated position isthus (Px, Py). The above equations are examples only, and otherequations used in conjunction with other screens may also generate acomparable result.

In FIG. 6 is shown a touch 601 over a plurality of stripes creating adistribution of Ct over said stripes. The resultant charge flows in theelement set up an areal distribution of Ct couplings across multiplestripes, roughly in proportion to the adjoining surface areas of touchand stripes. The principle of superposition applies (as it does in anysheet element) and the resultant determination of position will beproperly weighted and hence located to a far higher effective resolutionthan the number of stripes would seem to indicate. This effect is usedto greatly improve resolution in many other stripe based 2DCTs, forexample in U.S. Pat. No. 4,733,222 (Evans), but whereas Evans usesnumerical interpolation, the instant invention uses the physicalproperties of distributed capacitance among multiple stripes to achievethe same thing, without the need for further computation or for need forindividual electronic addressability of each stripe. The interpolationis intrinsic to the element itself. This is an effect previously knownto occur in 2DCT resistive sheet elements.

FIG. 7 a shows the calculated position along the X axis by Y row oftouch as calculated from the corner electrode signals according to Eqns1 and 2 above for a series of touches made at different X locationsbetween X=0.5 (center) and X=1 (right edge) along nine rows R0 to R8corresponding to nine Y locations between Y=0.5 and Y=1. FIG. 7 bschematically represents the distortion over all quadrants of thesensing element. A finger touching the element, whose circumference ofcontact encompasses a fractional number of stripes, shows no cogging ornonlinearity in X worth noting when dragged orthogonally to the stripes.This can be seen more graphically in FIG. 8 which shows a plot of thecorrection vectors for 7 rows cutting across the stripes in X. At noplace is there a non-orthogonal, non-vertical correction vector.

This remarkable result comes about because the stripes restrict coregalvanic current flows to the stripes, which lie only along the Y axis;this restriction prevents non-orthogonal current vectors anywhere in theelement. Once the current flows reach a bus-bar from a stripe, the flowrotates 90 degrees and heads towards the two nearest corner electrodes.It is only at this stage that the currents can be diverted down adjacentstripes to electrodes on the second bus-bar. This creates the pincushion effect along the bus-bars.

FIGS. 7 a, 7 b, and 8 show that in the Y axis the distortions are linearand can be corrected using a simple scaling factor which has adependency on X. For each position in X, there exists a single scalar(non-vector) correction factor which can be use to arrive at a correctedposition of touch in Y:Pcy _((x,v)) =P _(Y ↑(x))   (Equation 3)Where Py is the raw reported position in Y, Pcy(x,y) is the correctedposition for Y as a function of X and raw Y, and

(x) is a correction factor unique to each position in X for which acorrection factor is sought. The coefficients

(x) need only be solved for in any one quadrant (for example thequadrant represented in FIG. 7 a), and the results mirrored for theother 3 quadrants. The fact that there is no non-orthogonal component tothe correction, and that a single factor

(x) applies to any signal in Y(x), simplifies computations by two ordersof magnitude over Babb so that very rapid compensation can be performedusing slow, cheap microcontrollers for example costing under US$0.50.Furthermore the simplicity of the distortion and the correction methodimply that the element is also more stable under fluctuating temperatureor electrical conditions and is more repeatable to manufacture. UnlikeBabb, the correction of the element does not require multiple trials toallow curve fitting. So long as the strip-to-bus-bar resistance ratio isrepeatable (absolute stability is not required), the factors

(x) will be the same from one unit to the next. Inconsistencies fromunit to unit will have only a limited effect on the error term inreported touch location, and errors on one axis will create only highlyattenuated errors on the other axis. The element of the inventiongenerally isolates error terms between X and Y, a non-trivial beneficialeffect compared with the prior art.

The simplicity of the instant invention should be compare with the ‘80coefficients’ and fourth order polynomials required for Babb, thecoefficients of which must be determined though an extensive calibrationprocedure. The instant invention may require only single pointcalibration, or in most cases no calibration at all, as elementdistortions are simple, predictable, and repeatable from unit to unit.

The

(x) correction factors can be applied by means of a lookup table withinterpolation to achieve a simple, fast correction. The correctionfactor

(x) can also be computed mathematically using the simple quadraticequation: $\begin{matrix}{\quad{\left. \updownarrow(X) \right. = \frac{1}{{k_{1}X^{2}} + {k_{2}X} + k_{3}}}} & \left( {{Equation}\quad 4} \right)\end{matrix}$Leading to the complete equation for Y correction: $\begin{matrix}{{Pcy}_{({x,y})} = \frac{P_{Y}}{{k_{1}X^{2}} + {k_{2}X} + k_{3}}} & \left( {{Equation}\quad 5} \right)\end{matrix}$Where k1, k2 and k3 are coefficients that depend on the curvature ofpin-cushion distortion, and X is the absolute magnitude of the positionalong the X axis starting from center-screen and moving in either theleft or right direction. This quadratic equation was derived fromsimulation models and is accurate to better than 1%. It does not accountfor gross material nonlinearity which can be compensated for usingsecondary methods if required. The equations are dependent on resistanceratios between the bus-bars and the stripes as well as the geometricproportions of the element. The equations are unaffected by absoluteresistance values.

The analysis supra applies equally to a 2DCT or a 2DRT. A 2DRT generallyoperates in ‘reverse’ to a 2DCT in that the element is only driven bysignals, which are then picked off by a coversheet using a 5^(th)electrode connection for analysis purposes. The electrodes on theelement proper are usually driven in a time-multiplexed mode so as toallow for unique signals to be picked up in alternating X and Ydirections. For example the two left electrodes are first grounded, andthe two right ones driven to a fixed and identical potential; the coversheet is sampled to obtain a raw X position. The bottom electrodes arenext grounded, and the top electrodes connected to a fixed and identicalpotential; the cover sheet is sampled to obtain a raw Y position. Theprocess is repeated continuously, and a sample is declared valid only ifthe cover sheet is sensed to be in galvanic contact with the element.This is a potentiometric pickoff method, well described in the patentliterature. Other 2DRT sampling methods are possible and the sequencenoted in this paragraph should not be considered a preferred method, noris the sampling method an object of the invention.

Equation 5 needs only a set of solutions in one quadrant, with theresults mirrored for the other 3 quadrants. This is demonstrated in FIG.7 a and 7 b. FIG. 7 a shows the distortion in the top right quadrant;this pattern is mirrored in the other three quadrants to create thepattern of 7 b.

Handshadow; Zonal 2DCT Element

The phenomenon of 2DCT handshadow is described in my U.S. applicationSer. No. 10/341948 ( published as US20030132922) and in U.S. Pat. No.5,457,289. Screens that are ‘mobile phone size’ such as 60×60 mm, willnot generally suffer sufficiently from handshadow to warrant correctiveaction. However, if desired, one way to reduce the effects of handshadowis described in my aforesaid U.S. application Ser. No. 10/341948.

A second method involves essentially repeating the element of theinvention a second time as shown in FIG. 9. However, as can be seen fromFIG. 9, this can be achieved by effectively sharing a bus-bar to reduceassociated component counts. When the pattern is excited bydriver/sensor circuitry on the 6 nodes (i.e. electrodes) 301, 302, 301a, 302 a, 303 and 304 as shown, the element is effectively divided up ina way that allows it to be sensed in two different zones, top andbottom. Sensing within these zones is as described above. As handshadowcapacitance occurs primarily below the point of touch, a touch in theupper zone will cause handshadow primarily in the lower zone, where itcan be ‘processed away’ by simply ignoring the signals from said lowerzone. There is very little cross-coupling of handshadow currents betweenzones.

Larger screens would make use of even more of these zones, to a numberappropriate to the vertical size of the entire element and according tothe severity of the problem.

2DRT Application

FIG. 10 shows an element having slits between stripes. Such a method ofseparating stripes is particularly useful for 2DRT usage, where a coversheet is deflected to contact the element at a small point. If the pointof contact is smaller than the gap between stripes 310, it can be thatthe coversheet fails to pick up a potential and the contact fails.

FIG. 11 shows a resistive screen according to the invention, wherein acover sheet picks off a galvanic potential from the slit stripes of FIG.10 when bent inwards via touch or via a stylus. The element of FIG. 11is exactly as described above for a 2DCT, but the operating mode isaccording to various 5-wire screen modes as discussed in other patentsand in the open literature. Normally the cover sheet is held apart fromthe element via tiny ‘microbump’ spacers (not shown), as is well knownin the art.

Minimalist 2DCT

FIG. 12 shows a minimal 2DCT case, where there are 2 bus-bars and twostripes, all on the periphery of an element. The element is of a sizenot significantly larger than the object being sensed, so that thesignal levels in the middle do not significantly diminish due todistance from object to each conductive member. This example operateswithout measurable pin cushion as the impedances of the stripes andbus-bars are many orders of magnitude lower than the capacitive couplingimpedance from object to any strip or bus-bar. In this minimal case, thestripes and bus-bars can have the same value or wildly differing valueswith minimal observable effect on resolution or linearity. If theelement of FIG. 12 is for use by a human finger, the element shouldpreferably be no more than 4 times as wide or high as the diameter of afinger in order to provide reasonable signal strengths. The element ofFIG. 12 can be used to create a ‘mini mouse pad’ or pointer control, forexample for use by those with minimal appendage mobility, whereby verysmall motions of a fingertip or other appendage control an appliance orGUI.

Point-Screen 2DCT Operation

The 2DCT element is suited to use in a ‘point mode’ where the usersimply points at the screen. The easily correctable pin cushion and useof a single element mean that fields are not localized to shortdistances. As a result, the invention can be used as a ‘point screen’device with reasonable accuracy in most menu-based graphical interfaces.

This mode of operation can be extremely beneficial in hygieneapplications such as in hospitals, but also in ordinary consumer usagemodes to prevent screen smudging.

2DCT Drive Circuitry

Refer now to FIG. 13, wherein is shown preferred (but not essential)drive circuitry for the 2DCT application of the invention. This circuitis of the same type as shown in my co-pending U.S. application Ser. No.10/697133 but applied to all 4 electrodes (or 6 electrodes in the caseof FIG. 9, etc). The repeated switching of switches 1302, 1303, 1304 atlocations A, A′, A″, A′″, B, B′, B″, B′″, and C, C′, C″, C′″ areperformed simultaneously at each electrode so as to inject and measurecharge using four capacitors, also referred to as sampling capacitors,Cs, 1305, at equal moments in time. This is performed via switchcontroller 1307. Signal outputs are the tabulated number of switchingcycles for each electrode required to exceed a threshold voltage Vt, asdetermined by a voltage comparator 1301. The tabulation of cycle countsfor each electrode is performed by four counters at 1306.

The operation of this circuitry is explained more fully in my U.S.application Ser. No. 10/697133, incorporated by reference herein.

The invention can alternatively employ any of the switching sequencesand topologies as described in my U.S. Pat. No. 6,466,036, incorporatedherein by reference.

Signal processing circuitry is shown in FIG. 14, wherein the fourelectrode signals are input to the processing circuitry which in turncomputes a coordinate result. A logic block, microcontroller, or otherhardware or software is used to perform the calculations necessary toachieve the desired output. The block of FIG. 14 is usually a part ofanother system, such as a personal computer, process controller,appliance and so on, and the output may be only an intermediate resultin a larger process.

FIG. 15 shows a preferred embodiment of the invention wherein a singlemicrocontroller 1501 performs the switching functions of FIG. 13, plusperforms the signal processing of FIG. 14. The switching functions canbe performed in software on a conventional I/O port, or with an on-chiphardware capacitive conversion peripheral. Signal processing isperformed in software to achieve the desired coordinate output. Thisoutput could be a mere intermediate result used to control a largerprocess, and the output shown may only exist as numbers inside the chip.

Alternatively the invention can use any capacitive or resistive sensingcircuit described in literature. The gradient response of the element isnormally the same regardless of the type of drive circuitry. Theinvention is not reliant on any one acquisition method.

Materials, Fabrication

The 2DxT element is preferably made of a clear conductor of suitableresistance on the back of a glass or plastic sheet covering the display,if a touchscreen, or over a suitable dielectric substrate if a mousepad,etc.

As described in various other patents, the need for low-R bus-bars(under about 200 ohms end-to-end) on the edges causes all manner ofdriving, power, stability, and repeatability issues. It is highlydesirable to use materials with a much higher resistance than currentlyin widespread use. Most ITO (Indium Tin Oxide) layers such as thoseproduced by CPFilms, USA, or when custom-sputtered onto a surface, haveresistances around 300 ohms per square. It is highly desirable toelevate this resistance to the neighborhood of 500 to 2000 ohms persquare, so that the stripe and bus-bar resistances can be made in theregion of 25K ohms and up from end to end.

One method of increasing bus-bar and stripe resistance from lowresistance materials is to use a meandering path or zig-zag pattern soas to increase track length. Conventional low-resistance ITO orTin-Oxide coatings can be etched or patterned to have intentional voids(‘Swiss cheese’ approach), thus raising the resistance. The stripes andbus-bars can also be made suitably thin so that the resistance is highenough to be more optimal.

Ideal materials however will have an intrinsic resistance of about 500to 1,000 ohms per square or more, or can be modified to become so.Agfa's Orgacon™ conductive polymer is one material that has such a highintrinsic resistance and is also clear, making it usable intouch-screens over displays. A particularly low cost material is carbonbased ink, well known in the electronics trade, however being opaquethis material is better suited for tablet or mousepad applications.

The above being said, there is no requirement for any particular elementresistance value, and the driving circuitry can be adapted to almostanything with varying degrees of difficulty. In theory the onlyrequirement is that the elements have a non-zero resistance. However thebus-bar resistances should preferably be comparable or lower in valuethan the aggregate parallel value of the stripes in order to reduceY-axis pin cushion. A greater number of stripes would generally mean ahigher resistance per stripe to achieve the same effect, the pin cushionbeing related to the total bridging resistance between stripes, thebridging resistance being the parallel equivalent value of the stripes.Stripes located towards the center of the bus-bars have adisproportionate effect on pin cushion.

Patterning of the element into bus-bars and stripes can be via vapordeposition using a suitable stencil to prevent unwanted areas ofcoating, or via silk screening to create the desired pattern, or viapad-printing, or via laser scribe or chemical etching or chemicalreaction, or any other process which can create a patterned layer. Inthe case of conductive polymer Agfa Orgacon™, the pattern can be createdby using sodium hypochlorite to force areas to become non-conductive viachemical reaction without actual material removal.

Fabrication can entail the use of normal touchscreen or touchpad methodssuch as vapor deposition of appropriate materials onto a glass sheetplaced in front of a display.

In-mold decorating (“IMD”) entails the use of a graphic sheet or layerplaced inside the injection mold or cast prior to the introduction offluid plastic. Once molded, the layer becomes an integral part of theresultant plastic piece. In the case of a 2DCT, a conductive element ofthe type according to the invention is placed in the mold for a displaycover; when injected, the conductive layer becomes fused to one side ofthe cover. In this way complex cover shapes, including those havingcompound curves, can be created which include an integral 2DCT atextremely low cost.

Electrode connections can be made via wires bonded to the corners, orvia conductive rubber pillars, or using metal springs, etc. Conductiverubber is a method of choice for very low cost connection from anunderlying PCB containing the driver circuitry. FIG. 16 shows such aconstruction method in cross-section. Display 1601 is viewed throughcover lens 1602 and sensing element 300. Element 300 is connected via atleast four corner electrodes through conductive rubber posts, of whichtwo 1603 a, 1603 b are shown, to PCB 1604. The entire assembly is placedunder compression via screws, clamps or other fastener system (notshown) so that the rubber posts are compressed and thus forced to makecontact between PCB 1604 and element 300.

The element can also be fabricated from molecular substances havinganisotropic conduction. For example, a conductive polymer can beenvisioned having conductivity that is much better in one direction thananother. Such materials based on nanostructures have been described inthe literature, for example in literature from Helsinki University ofTechnology.

2DxT Stripe Weighting

One embodiment of the invention weights the stripes so that the onesnear center-screen are either spaced further apart or have a higherresistance or both. The effect of this is to reduce the amount ofinherent pin cushion. However, tests have shown that while this in factis what happens, it also means that there will be a loss of signal inthe center of the element, and/or there will be drive problems throughthe resultant higher resistance and lower finger coupling to neighboringtraces. In practice this approach is not deemed to be efficacious, andis mentioned here only for completeness.

2DCT Acquisition Manipulation

Problems associated with 2DCT's include interference from outsideelectrostatic or radio sources having a frequency at the operatingfrequency of the element, or some harmonic thereof. These problems canbe attenuated by using a modulated operating frequency for the signalacquisition so as to reduce or prevent signal-noise aliasing or beating.This can involve the use of frequency hopping, chirps, or pseudo-randomfrequency modulation. These methods are known as ‘spread-spectrum’modulation.

Post processing can include the use of majority vote filtering, medianfiltering, averaging, and so on to reduce the residual effects of noisethat are already attenuated by means of the frequency modulation.

Low frequency interference can be caused by local mains fields and soon. This form of interference can be attenuated by synchronizing theacquisition to the interfering source, for example 50 or 60 Hz, asdescribed in the datasheet for the Quantum Research Group Ltd (UK) QT310device.

2DCT Driven Shield

The element is compatible with driven shield methods to reduceinterference from LCD displays, VFD switching, etc. This entails the useof a conductive plane behind the element positioned between the elementand the interfering source. A drive shield can also protect againstsignal disturbance from motion behind the element. Driven back-shieldsare commonly used in the construction of 2DCT's.

2DCT Wake-Up

In many applications it is desirable to have a ‘wakeup’ function,whereby the entire device ‘sleeps’ or is in some quiescent or backgroundstate. In such cases, it is often desirable to have a wake signal frommere proximity of a human body part some distance away.

The element can be driven as a single large capacitive electrode withoutregard to position location, while the unit is in the background state.During this state the electronic driver logic looks for a very smallchange in signal, not necessarily enough to process as a 2D coordinate,but enough to determine that an object or human is in proximity. Theelectronics then ‘wakes up’ the overall system and the element is drivenso as to become a true 2DCT once again.

Tablet, Mouse Pad Usage; Injection Mode

The element of the invention in 2DCT mode is suitable as a mouse pad, oras a tablet type input device. In these roles, there is no need foroptical transparency. A stylus can be used with the element either topick up a radiated electric field from the element, or to inject asignal into the element, or to act as a human finger.

In injection mode, the element of the invention merely operates inreverse. A signal from a tethered pen is injected capacitively into theelement in a region surrounding the point of contact. The signal is thenapportioned ratiometrically to the four corner electrodes, from whenceit can be picked up and conveyed to a measurement circuit of almost anytype already described in literature and then processed to create anindicative result. The pin cushion result operates in substantially thesame way in injection mode as it does in a 2DRT or 2DCT mode; the vectorgradients are the same.

2DxT Uncorrected Mode

Many applications do not require linearization of the result. These areprincipally those applications involving human interfaces of lowresolution, for example for menu button sensing and the like. In suchapplications, the element and related signal acquisition circuitry candispense with the linearization step and simply generate the raw output.Additional system logic would interpret 2D coordinate boundaries asbeing touch buttons, the boundaries being defined at the time ofsoftware development.

If the ratio of stripe-to-bus-bar resistance is high enough, theaccuracy of the raw processed result may be acceptable for direct use.For example, if the resulting coordinate error of an element is only 5%but is repeatable, the element may be perfectly suitable for uncorrectedmenu button detection over a display where the buttons do not occupyless than 10% of the height of the element. If the buttons areprincipally located near the horizontal centerline of the element, oralong the left or right sides, the distortion could be negligible and ifso, no linearization correction need be applied.

FURTHER EXAMPLES

FIG. 17 schematically shows a pattern of conductive material used in asensing element 1702 according to another embodiment of the invention.The pattern differs from the pattern shown in FIG. 3 in that theconductive material of the sensing element shown in FIG. 17 is arrangedin a zig-zag pattern. The sensing element 1702 is otherwise similar toand may be operated in the same manner as the sensing element shown inFIG. 3. Thus the sensing element 1702 comprises a pair of spaced apartbus bars 1706, 1708 formed from conductive material arranged in azig-zag pattern with a series of similarly zig-zagged stripes 1710 (alsoknown as strips) connecting between them to provide the anisotropicconductive area. Electrodes 1712 a-d (which are similar to and will beunderstood from the electrodes 301-304 shown in FIG. 3) are provided atthe corners of the pattern of conductive material.

By depositing (or etching) zig-zag patterns of the kind shown in FIG.17, the effective resistance per unit length of the bus bars and stripescan be increased. This can be particularly useful in smaller screenwhere the resistance of the bus bars and/or stripes may otherwise beundesirably low. Triangular zig-zagging such as shown in FIG. 17 is onlyone example of suitable patterning. Other patterns in which theintegrated length of conductive material along at least a section of thebus bars and/or stripes connecting between two points is longer than thestraight-line distance between the points could also be used. Forexample square zig-zagging or a sinusoidal patterning (e.g. similar torounded triangle zig-zagging) could be used to provide the same effect.

FIG. 18 schematically shows a pattern of conductive material used in asensing element 1802 according to another embodiment of the invention.The pattern shown in FIG. 18 is similar to and will be understood fromthe pattern shown in FIG. 17 in that it employs a form of zig-zagging toincrease the resistance provided by the bus bars and stripes between twopoints on the sensing element. However, although the sensing element1802 operates in fundamentally the same way, the specific patterning isdifferent with the more open triangular zig-zag pattern of FIG. 17replaced with an more closely packed and interleaved square zig-zagpattern in which the resistive paths double-back on themselves severaltimes. Also, whereas the anisotropic conducting area of the sensingelement 1702 of FIG. 17 is provided by ten stripes, the anisotropicconducting area in the example shown in FIG. 18 is provided by fivestripes 1810. In FIG. 18 (unlike FIG. 17) the conductive material isshown white and the non-conducting regions which separate the stripes isshown in black.

The sensing element 1802 comprises a pair of spaced apart bus bars 1806,1808. The pattern of conductive material is such that the bus barsregularly double-back on themselves to increase their effectiveresistive lengths. With the arrangement shown in FIG. 18, the resistivelength of the bus bars is around three times longer than the distancethey span on the sensing element. By doubling-back in this way, e.g. asopposed to simply employing thinner (and hence more resistive) straightline resistive paths, increased resistance can be provided withoutsignificantly reducing the overall surface area of the electrodes forcoupling to an object whose position is to be determined. The regions ofthe conductive material corresponding to the bus bars 1806, 1808 areidentified in FIG. 18 by cross-hatching.

The five stripes 1810 connecting between the bus bars 1806, 1808 toprovide the anisotropic conductive area are similarly arranged todouble-back on themselves. Thus the path length of the stripes issignificantly longer than the straight-line distance between the busbars. The central stripe 1810 is shown shaded in FIG. 18. The stripeseither side of the central stripe are arranged to follow a similarlyshaped pattern to the central stripe. However, these adjoining stripesare slightly offset in a direction perpendicular to the bus bars 1806,1808 so that the stripe patterns may be interweaved/interleaved with oneanother. The outer stripes are of different shape in that they followand interleave with their respective adjoining stripes on one side, butthe doubling back is curtailed on the outer side of the sensing elementto provide a flat edge.

The interdigitation of neighboring stripes provided by the interleavingpatterning in FIG. 18 can help provide a more uniform response toposition as it provides for more distributative coupling between anobject whose position is to be sensed and different ones of the stripes1810 and so helps to reduce apparent “cogging” as an object moved frombeing over one stripe to being over another.

As in the other embodiments, electrodes 1812 a-d are provided at thecorners of the pattern of conductive material to allow for positiondetermination as described above. However, the sensing element 1802shown in FIG. 18 further includes an additional guard electrode 1804.The guard electrode 1804 may be formed of the same material and in thesame way and at the same time as the conductive material providing theresistive bus bars and the anisotropically conducting area. Similarguard electrodes may be employed with the other embodiments of theinvention described above. The guard electrode 1804 can be particularlybeneficial for capacitive based position sensors as it can be coupled tocapacitance measurement circuitry similar to the capacitance measurementcircuitry coupled to the corner electrodes 1812 a-d and used to providea reference signal. The reference signal from the guard electrode 1804can be used to provide a moving offset measurement for the cornerelectrodes. Thus changes in the measured signals from the cornerelectrodes attributable to environmental conditions, as opposed to thepresence of an object whose position is to be sensed, can be morereadily accounted for.

Furthermore, signals from the guard electrode 1804 may be used to governthe way in which a device operated by position estimates from thesensing element responds to activation of the sensing element. Forexample, the device may be configured to ignore (or not to make)position estimates if the signal from the guard electrode indicates astrong degree of capacitive coupling to an object. This kind of“locking” can be useful to prevent unintentional activation of thedevice. For example, the position sensing element may be used in amobile telephone (cellular telephone) in which different region of thesensing area corresponding to the different buttons of a numeric keypad.In this case, when a user holds the telephone in his hand and pressesareas of the sensing element corresponding to the numeric buttons todial a number, the position sensor responds normally. However, when theuser holds the telephone against his head, the relatively large areaguard electrode is provided with a large degree of capacitive couplingdue to the close proximity of the user's head so that a positivedetection signal from the guard electrode is obtained. This signal canbe used to inhibit the position sensing part of the sensing element sothat numbers on the keypad are not accidentally activated, e.g. bybrushing against the users' ear while the telephone is adjacent hishead.

FIG. 19 schematically shows a pattern of conductive material used in asensing element 2002 according to another embodiment of the invention.The pattern is similar to that of FIG. 3, although it includes ninestripes 2010, 2011 instead of 10. Furthermore, the sensing element 2002of FIG. 19 is shown in an orientation that is rotated by 90 degreescompared to that of FIG. 3. Thus the sensing element 2002 of FIG. 19comprises a pair of spaced apart bus bars 2006, 2008 formed fromconductive material and a series of nine stripes 2010, 2011 connectingbetween the bus bars to provide an anisotropic conductive area.Electrodes 2012 a-d are provided at the corners of the pattern ofconductive material as in FIG. 3. However, two further electrodes 2012e, 2012 f are provided midway along respective ones of the bus bars2006, 2008. The electrodes may be coupled to drive channels similar tothose coupled to the other electrodes to provide a further pair ofoutput signals. As in the other embodiments, the various drive channelsmay be provided by separate drive circuits or by appropriatemultiplexing of a single drive circuit.

Thus the sensing element 2002 shown in FIG. 19 may in effect beconsidered as comprising two sensing elements which are adjacent oneanother and share a common stripe between their respective bus bars anda common pair of electrodes at the ends of their common stripe. That isto say the sensing element 2002 may be considered to comprise a lowersensing element corresponding to the lower half of the sensing element2002 (below the imaginary dashed line in FIG. 19) and an upper sensingelement corresponding to the upper half of the sensing element 2002(above the imaginary dashed line in FIG. 19).

The lower sensing element is cornered by the four electrodes labelled2012 e, 2012 f, 2012 c and 2012 d. The section of the bus bar 2006 whichlies between the electrodes labelled 2012 e and 2012 c provides one busbar for the lower sensing element, and the section of the bus bar 2008which lies between the electrodes labelled 2012 f and 2012 d providesanother bus bar for the lower sensing element. The central stripe 2011and the other of the stripes 2010 below it provide the stripes of thelower sensing element. Output signals from drive channels associatedwith the four electrodes labelled 2012 e, 2012 f, 2012 c and 2012 d maybe used to determine position within the lower sensing element.

The upper sensing element is cornered by the four electrodes labelled2012 a, 2012 b, 2012 e and 2012 f. The section of the bus bar 2006 whichlies between the electrodes labelled 2012 a and 2012 e provides one busbar for the upper sensing element, and the section of the bus bar 2008which lies between the electrodes labelled 2012 b and 2012 f providesanother bus bar for the upper sensing element. The central stripe 2011and the other of the stripes 2010 above it provide the stripes of theabove sensing element. Output signals from drive channels associatedwith the four electrodes labelled 2012 a, 2012 b, 2012 e and 2012 f maybe used to determine position within the upper sensing element.

Position estimates derived from the upper and lower sensing elementsseparately can be used to give an indication of the distribution of thecoupling of an object to the sensing element. For example, significantcoupling to both the upper and lower sensing elements may indicate asignificant degree of handshadow is present.

FIG. 20 schematically shows the sensing element 2002 of FIG. 19 beingused to detect the position of a finger 2005 associated with a hand 2007in a situation where there is significant handshadow. The user intendsto indicate a position beneath his finger, i.e. in the region of strongcapacitive coupling indicated in FIG. 20 by the letter T. However, theuser's hand also presents a larger area of less strong (because it isfurther away) capacitive coupling marked in FIG. 20 by the letter H. Ifthe position of the touch is calculated solely from the signals from thefour corner electrodes 2012 a, b, c and d, the inherent interpolationprovided by the sensing element will lead to an output result whichrepresents the centroid of the coupling regions T and H, which might,for example, be the point indicated by the letter R. However, by takingaccount of the signals associated with the further electrodes 2012 e(obscured by hand in FIG. 20) and 2012 f, separate position estimatescan be derived from the upper and lower halves of the sensing element2002.

Thus the signal from the upper half of the sensing element 2002 can beused to provide an estimate of position which is not effected by thecapacitive coupling associated with the hand 2007 since this isprimarily to the lower half of the sensing element. In general, if theorientation of the sensing element in use is such that a user's handwill normally approach it as shown in FIG. 20, if there is significantcoupling to both the top and bottom halves of the sensing element 2002,the user's finger might be assumed to be in the top half, and his handin the bottom half. In effect, the lower stripes are absorbing theeffect of the handshadow to prevent mixing of the signals associatedwith the user's hand with those associated with his finger. If, there isonly significant coupling to the bottom half, the user's finger might beassumed to be in the bottom half, the remainder of his hand being belowthis and so outside the sensitive area of the sensing element.

Accordingly, the arrangement is in effect the same, and provides thesame benefits (e.g. rejection of handshadow signals), as the arrangementshown in FIG. 9 but is based on sharing a common edge stripe and itsassociated electrodes as opposed to a common bus bar and its associatedelectrodes. The inventors have found that the arrangement shown in FIG.19 can lead to improved handshadow rejection over the arrangement shownin FIG. 9.

FIG. 21 schematically shows a pattern of conductive material used in asensing element 2202 according to another embodiment of the invention.The pattern in similar to that of FIG. 3 but is shown in an orientationthat is rotated by 90 degrees compared to that of FIG. 3. Thus thesensing element 2202 of FIG. 19 comprises a pair of spaced apart busbars 2206, 2208 formed from conductive material and a series of tenstripes 2210, 2211, 2213 connecting between the bus bars to provide ananisotropic conductive area. Electrodes 2212 a-d are provided at thecorners of the pattern of conductive material as in FIG. 3. However,four further electrodes 2212 e, 2212 f, 2212 g, 2212 h are provided aspairs along respective ones of the bus bars 2206, 2208. These furtherelectrodes may again be coupled to drive channels similar to thosecoupled to the other electrodes to provide a further four outputsignals. As in the other embodiments, the various drive channels may beprovided by separate drive circuits or by appropriate multiplexing of asingle drive circuit.

Thus the sensing element 2202 shown in FIG. 19 may in effect beconsidered as comprising three sensing elements. That is to say thesensing element 2202 may be considered to comprise a lower sensingelement corresponding to the lower third of the sensing element 2202(below the lower imaginary dashed line 2215 in FIG. 19), a middlesensing element corresponding to the middle third of the sensing element2202 (between the imaginary dashed lines 2213, 2215 in FIG. 19), and anupper sensing element corresponding to the upper third of the sensingelement 2202 (above the imaginary dashed line 2213 in FIG. 19).

The lower sensing element is cornered by the four electrodes labelled2212 g, 2212 h, 2212 c and 2212 d. The section of the bus bar 2206 whichlies between the electrodes labelled 2212 g and 2212 c provides one busbar for the lower sensing element, and the section of the bus bar 2208which lies between the electrodes labelled 2212 h and 2212 d providesanother bus bar for the lower sensing element. The stripe 2213 and theother of the stripes 2210 below it provide the stripes of the lowersensing element. Output signals from drive channels associated with thefour electrodes labelled 2012 g, 2012 h, 2012 c and 2012 d may be usedto determine position within the lower sensing element.

The middle sensing element is cornered by the four electrodes labelled2212 e, 2212 f, 2212 g and 2212 h and the upper sensing element issensing element is cornered by the four electrodes labelled 2212 a, 2212b, 2212 e and 2212 f. Output signals from the drive channels associatedwith these electrodes may be used to determine position respectivelywithin the middle and upper sensing element.

Thus the use of eight electrodes as shown in FIG. 21 allows positionestimates to be derived from the upper, middle and lower sensingelements. These can be used to give an indication of the distribution ofthe coupling of an object to the sensing element in the same way asdescribed above. However, by providing three, as opposed to two,position estimates, a better measure of the distribution of the couplingis provided and so more reliable handshadow rejection can be provided.

Still more electrodes could be used so as to in effect divide thesensing element up into more sections, e.g., ten electrodes may be usedto divide it into four section, twelve electrode for five sections, andso on.

Features of the other embodiments, such as the zig-zagging,interleaving/interweaving and guard electrodes of FIGS. 17 and 18, couldalso be used with the sensing element of FIG. 19.

Summary

The invention is at its basic reduction, an element whose purpose is toprovide for an improved form of 2D sensing device via anisotropicconduction, plus, optionally, a method to correct the distortions of theraw computed coordinate result. The mode of operation (including butwithout limitation, galvanic or capacitive modes), the use to which itis put, and whether it is used as a receiver of signals from a stylus ora sensor of passive touch is not of prime importance to the invention.What is important is the anisotropic structure of the element and theform of positional error it produces, and the optional methods disclosedherein for correcting the error.

An important aspect of sensing elements of embodiments of the inventionis that they can be made as a single-layer having a core that conductswell galvanically in a first predetermined direction, but suppressesconduction in a second direction orthogonal to the first, i.e. it hasanisotropic conductivity, plus, the core is bounded by a resistiveborder to make the whole element. The element furthermore has fourelectrodes in the corners and which are driven and/or sensed by anelectronic circuit to create a resulting output indicative of touchposition.

There are many variations possible as will become evident to thoseskilled in the art, involving various combinations of detection methodsor switch sequences outlined specifically herein. The methods disclosedherein can be combined with other methods as taught in any number of myprior patents including methods for drift compensation, calibration,moisture suppression using short switch closure times, and the like.Particular note should be made of the various possible combinations offeatures disclosed in my own prior art involving capacitive sensingmethods, all of which are incorporated herein by reference; also notethe capacitive products as described in the datasheets of QuantumResearch Group Ltd (UK), many of which have features germane to theinstant invention.

It is also possible to warp the invention into unusual shapes asdisclosed by Pepper. Such transformations may prove useful in objectposition sensing for example in industrial settings, where locationalong a cylinder, sphere, or other curved surface might be important.

It will be appreciated that although particular embodiments of theinvention have been described, many modifications/additions and/orsubstitutions may be made within the spirit and scope of the presentinvention.

1. A touch sensitive position sensor comprising: a substrate defining atouch sensitive platform; first and second resistive bus-bars arrangedspaced apart on the substrate; and an anisotropic conductive area havingnon-zero resistance arranged between the bus-bars such that currentsinduced in the anisotropic conductive area flow preferentially towardsthe bus-bars.
 2. A touch sensitive position sensor according to claim 1,wherein the first resistive bus-bar extends between a first and a secondelectrode and the second resistive bus-bar extends between a third and afourth electrode, the position sensor further comprising first, second,third and fourth drive channels associated with respective ones of thefirst, second, third and fourth electrodes, each drive channel beingoperable to generate an output signal dependent on the resistancebetween its electrode and the position of the object.
 3. A touchsensitive position sensor according to claim 2, further comprising afifth electrode coupled to the first resistive bus-bar at a locationbetween the first and second electrodes, a sixth electrode coupled tothe second resistive bus-bar at a location between the third and fourthelectrodes, and fifth and sixth drive channels respectively associatedwith the fifth and sixth electrodes, wherein the fifth and six drivechannels are operable to generate output signals dependent on theresistance between their respective electrode and the position of theobject.
 4. A touch sensitive position sensor according to claim 3,further comprising a processor operable to generate an estimate for theposition of the object by comparing the output signals from the fifthand sixth drive channels and the output signals from a selected pair ofthe output signals from either the first and second drive signals or thethird and fourth drive signals.
 5. A touch sensitive position sensoraccording to claim 4, wherein the processor is configured to estimatethe position of the object in a first direction running between thebus-bars from a ratiometric analysis of the sum of the signalsassociated with the fifth and sixth electrodes and the sum of thesignals associated with the selected pair of the other output signals.6. A touch sensitive position sensor according to claim 4, wherein theprocessor is configured to estimate the position of the object in asecond direction running along the bus-bars from a ratiometric analysisof the sum of the signals associated with the fifth and the one of theselected pair of the other output signals associated with the firstresistive bus bar, and the sum of the signals associated with the sixthand the one of the selected pair of the other output signals associatedwith the second resistive bus bar.
 7. A touch sensitive position sensoraccording to claim 3 further comprising a seventh electrode coupled tothe first resistive bus-bar at a location between the first and secondelectrodes, an eighth electrode coupled to the second resistive bus-barat a location between the third and fourth electrodes, and seventh andeighth drive channels respectively associated with the seventh andeighth electrodes, wherein the seventh and eighth drive channels areoperable to generate output signals dependent on the resistance betweentheir respective electrode and the position of the object.
 8. A touchsensitive position sensor according to claim 1, further comprising afurther substrate defining a further touch sensitive platform adjacentthe first-mentioned touch sensitive platform; further first and secondresistive bus-bars arranged spaced apart on the further substrate; and afurther anisotropic conductive area having non-zero resistance arrangedbetween the further bus-bars such that currents induced in the furtheranisotropic conductive area flow preferentially towards the furtherbus-bars, wherein one of the first mentioned first and second resistivebus-bars and one of the further first and second resistive bus-bars area shared bus-bar.
 9. A touch sensitive position sensor according toclaim 1, further comprising: a third resistive bus-bar arranged on thesubstrate spaced apart from the second resistive bus-bar; and a furtheranisotropic conductive area having non-zero resistance arranged betweenthe second and third bus-bars such that currents induced in the furtheranisotropic conductive area flow preferentially towards the second andthird bus-bars.
 10. A touch sensitive position sensor according to claim1, wherein the resistive bus bars are configured to have a path lengthbetween two locations which is greater than the straight line distancebetween the two locations.
 11. A touch sensitive position sensoraccording to claim 10, wherein the resistive bus bars are configured ina zig-zag pattern.
 12. A touch sensitive position sensor according toclaim 10, wherein the resistive bus bars are configured to double-backon themselves multiple times.
 13. A touch sensitive position sensoraccording to claim 1, wherein the anisotropic conducting area is formedfrom a plurality of resistive strips connecting between the bus bars,and wherein the resistive strips are configured to have a path lengthwhich is greater than the straight line distance between the bus bars.14. A touch sensitive position sensor according to claim 13, wherein thestrips are configured in a zig-zag pattern.
 15. A touch sensitiveposition sensor according to claim 13, wherein the strips are configuredto double-back on themselves multiple times.
 16. A touch sensitiveposition sensor according to claim 13, wherein neighboring strips areconfigured to interleave with one another.
 17. A touch sensitiveposition sensor according to claim 2, further comprising a guardelectrode adjacent to the bus bars and/or the anisotropic conductingarea.
 18. A touch sensitive position sensor according to claim 17,wherein the guard electrode at least partially surrounds the bus barsand the anisotropic conducting area.
 19. A touch sensitive positionsensor according to claim 17, further comprising a guard drive channelassociated with the guard electrode, the guard drive channel beingoperable to generate a guard electrode output signal indicative of acoupling between an object and the guard electrode.
 20. A touchsensitive position sensor according to claim 19, further comprising aprocessor operable to compare the output signal from the guard electrodewith the output signals from at least one of the other electrodes tocompensate for changes in the output signal from the at least one of theother electrodes which are not associated with an object whose positionis to be determined.
 21. A touch sensitive position sensor according toclaim 19, further comprising a processor operable to disregard outputsignals from the electrodes coupled to the resistive bus bars accordingto the output signal from the guard electrode.